"Brockman says the ChatGPT API is powered by the same AI model behind OpenAI’s wildly popular ChatGPT, dubbed “gpt-3.5-turbo.” GPT-3.5 is the most powerful text-generating model OpenAI offers today through its API suite; the “turbo” moniker refers to an optimized, more responsive version of GPT-3.5 that OpenAI’s been quietly testing for ChatGPT." [0]

Chinchilla optimization is a technique which can be applied to existing models by anyone, including OpenAI. The chatGPT API is not based on GPT-4.

[0] https://techcrunch.com/2023/03/01/openai-launches-an-api-for...